This application is a U.S. National Phase Application under 35 USC 371 of International Application PCT/JP00/06025 (not published in English) filed Sep. 6, 2000.
This invention relates to an optical time-domain reflectometer (hereinafter referred to as xe2x80x9cOTDRxe2x80x9d) that is use search for defective points in an optical fiber. More particularly, the invention relates to an OTDR that does not influence the communication via the line during measuring.
As is known, an OTDR is connected to one end of an optical fiber and applies a pulsed light beam into the optical fiber from the end thereof. The OTDR measures the time that lapses until reflected light (backscattered light) arrives from the optical fiber, the level of the reflected light and the like. The losses in the optical fiber, the positions where the losses occur (e.g., connection points), the positions of far ends, and the like are thereby determined.
FIG. 6 is a diagram showing how optical fibers are laid and how the various characteristics of the optical fibers are measured by a conventional technique.
On a panel 30 a port is provided for a plurality of optical fibers F. Of the fibers F, one line is used to achieve communication (i.e., an active line) is connected to a transmitter 32a and a receiver 32b. 
For convenience, only one core line of the optical fiber F, or active line, is illustrated in FIG. 6. In practice, many lines of the optical fiber F serve as active lines.
An OTDR 31 is connected to one of the optical fibers F that are not used, that is, those other than the active lines, in order to perform various measurements on that optical fiber F.
The core line that is used as an active line is known and identified with the core-line number and the like. The OTDR 31 is therefore connected to a port of a core-line number not assigned to the active line.
The core-line number may be mistaken for another and the measuring may be started. In this case, measuring light (pulsed light) is applied into the core of the active line through which communication is being achieved. This may influence the communication.
Also, the waveform of the measuring signal changes to an abnormal one. Consequently, normal measuring of characteristics may not be accomplished.
The panel 30 has hundreds of ports or thousands of ports. Almost all ports are in connected state, no matter whether they are used for active lines. To initiate measuring, the ports are disconnected and connected to the OTDR. Hence, one core-line number may be mistaken for another, as mentioned above.
Once the port of any active line is disconnected, the light being used is no longer applied, inevitably causing a problem. In this condition, any port that is not of any active line can be identified, only by the core-line number allocated to the port. This may cause a mistake.
In some cases, a measuring port 33 is provided at a position different from the position where the panel 30 is arranged.
The measuring port 33 is connected to each core line of the fiber F by means of a coupler (not shown) or the like. The port 33 is used during the measuring only; it is usually opened.
Once the OTDR 31 is connected to the measuring port 33 to effect measuring, the core line of the fiber F, which is connected to the transmitter 32a and receiver 32b, cannot be visually detected.
When light is applied to the core line of a wrong number, thereby starting the measuring, the measuring signal will immediately enter the communication line, adversely influencing the communication. In addition, the measuring side will be affected, resulting in an abnormal waveform and the like.
The present invention has been made in order to solve the problems described above. The object of the invention is to provide an OTDR which prevents various measuring of an active line effecting communication, which influences neither the communication line nor the measuring and which can therefore perform measuring in safety.
According to one aspect of the invention, there is provided an OTDR having:
a light-emitting element (2b) for emitting pulsating measuring light when driven;
branching means (3) for applying the measuring light emitted from the light-emitting element, to an optical fiber to be measured, and branching light supplied from the optical fiber; and
light-receiving means (11) for receiving the light branched by the branching means and detecting a level of the light received.
The OTDR further comprises processing means for detecting a level of the light supplied from the optical fiber after the OTDR is connected to the optical fiber, while not driving the light-emitting element, and for driving the light-emitting element only when the level of the light received is equal to or lower than a preset value.
According to another aspect of this invention, there is provided an OTDR comprising:
light source driving means;
a light source for emitting pulsating measuring light when driven by the light source driving means;
branching means for applying the measuring light emitted from the light source, to an optical fiber to be measured, and branching light supplied from the optical fiber;
light-receiving means for receiving the light branched by the branching means and detecting a level of the light received;
determining means for causing the light-receiving means to detect the level of light received while the light source is not emitting light and for determining that the optical fiber is not serving as a communication line, only when the level of light received is equal to or lower than a preset value; and
processing means for causing the light source driving means to drive the light source, thereby to start the measuring of the optical fiber, when the determining means determines that the optical fiber is not serving as a communication line.
According to a further aspect of the invention, there is provided a method of measuring an optical fiber by use of an optical time-domain reflectometer, the method comprising the steps of:
providing a light source that emits pulsating measuring light when driven by light source driving means;
providing branching means for applying the measuring light emitted from the light source, to an optical fiber to be measured, and branching light supplied from the optical fiber;
providing light-receiving means for receiving the light branched by the branching means and detecting a level of the light received;
causing the light-receiving means to detect the level of light received while the light source is not emitting light and determining that the optical fiber is not serving as a communication line, only when the level of light received is equal to or lower than a preset value; and
causing the light source driving means to drive the light source, thereby to start the measuring of the optical fiber, when it is determined that the optical fiber is not serving as a communication line.